Ink optimization and total ink reduction has become an important new feature in various color management systems. For example, at the recent 2010 Technical Conference for the IDEAlliance/IPA, several ink-optimizing products were compared (see Sharma et al., “IPA Ink Optimization RoundUP 2010,” white paper presented at the 2010 IDEAlliance/IPA Technical Conference). These products attempt to reduce the amount of CMYK ink with little or no loss in color reproduction accuracy. Hence, these products were compared both in terms of the amount of ink reduction that was achieved, as well as in the preservation of appearance of converted CMYK images.
Three ways in which the total or average CMYK ink can be reduced is by increasing the amount of black ink in substitution for a corresponding reduction of CMY ink (GCR), applying cut-back curves to the individual channels (essentially applying a scale factor that is <1.0), and by lowering the limit on the total amount of ink that can be applied at any one location (“total area coverage” or TAC). Historically, these calculations are performed in the device coordinate space of the printing system being optimized, for example CMYK. Additionally, modifications to the perceptual tone reproduction can be used to reduce the overall ink load and this may be utilized in combination with the methods described herein to achieve a given targeted average ink load.
An advantage of the historical approaches is that the mathematical functions applied are relatively simple. However, a disadvantage is that the volume of the color gamut can often be greatly reduced, particularly in the region extending from saturated primaries (CMY) and secondaries (RGB) to the black point of the color gamut (i.e., the darkest color that can be printed given the ink load restrictions imposed). An excessive narrowing below the girdle of the color gamut is sometimes referred to as the “tornado effect” and can be exemplified in FIG. 1, which compares a full color gamut 100 for a CMYK printing system to a TAC-limited color gamut 105 that has been restricted by a very low value of TAC. The severe concavities associated with the TAC limited color gamut 105 can often have a significant impact on the resulting image quality. Another disadvantage is that the color reproduction accuracy may be sacrificed when utilizing a device coordinate transform based approach rather than a colorimetric approach.
Various complex solutions can be proposed in order to avoid this effect in the case of GCR by utilizing a colorimetric GCR approach. For example, in U.S. Pat. No. 7,495,804, to Rozzi, entitled “Gamut-preserving color imaging,” which is incorporated herein by reference, teaches a colorimetric GCR method that includes tracing out a locus of colorant values having different K values that can all achieve a given value of L*a*b* in conjunction with different values of CMY, as described. However, this approach does not address the problem that is encountered when attempting to keep the total ink below a designated TAC level when all colorimetric solutions on the locus of colorant values are above that TAC limit.
There remains a need for a method to form color transforms for a color printing device that can provide aggressive reductions in average total colorant load without producing an unnecessary loss of image quality or perceptual color reproduction.